pmb.config.apkbuild_package_attributes: + triggers (MR 2202)

Do not only parse triggers, but also emit them in the packages dicts in
pmb.parse.apkbuild. This is the first half to fix pmaports issue 1690,
the other is in pmaports MR 3386.

pmb/config/__init__.py

@@ -593,6 +593,7 @@ apkbuild_package_attributes = {
     "provides": {"array": True},
     "provider_priority": {"int": True},
     "install": {"array": True},
+    "triggers": {"array": True},
 
     # UI meta-packages can specify apps in "_pmb_recommends" to be explicitly
     # installed by default, and not implicitly as dependency of the UI meta-